The present invention relates to the production of metal strip, typically steel strip, which has a corrosion-resistant metallic coating that contains aluminium-zinc-silicon-magnesium as the main elements, although not necessarily the only elements, in the coating alloy, and is hereinafter referred to as an “Al—Zn—Si—Mg alloy” on this basis.
In particular, the present invention relates to a hot-dip coating method of forming a metallic coating on a strip that includes dipping uncoated strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on the strip.
Typically, the composition of the molten Al—Zn—Si—Mg alloy comprises the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
Zn: 30 to 60%Si:0.3 to 3% Mg:0.3 to 10%BalanceAl and unavoidable impurities.
More typically, the composition of the molten Al—Zn—Si—Mg alloy comprises the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
Zn:35 to 50%Si:1.2 to 2.5%Mg1.0 to 3.0%BalanceAl and unavoidable impurities.
The composition of the molten Al—Zn—Si—Mg alloy may contain other elements that are present in the molten alloy as deliberate alloying additions or as unavoidable impurities. Hence, the phrase “Al—Zn—Si—Mg alloy” is understood herein to cover alloys that contain such other elements as deliberate alloying additions or as unavoidable impurities. The other elements may include by way of example any one or more of Fe, Sr, Cr, and V.
Depending on the end-use application, the metallic coated strip may be painted, for example with a polymeric paint, on one or both surfaces of the strip. In this regard, the metallic coated strip may be sold as an end product itself or may have a paint coating applied to one or both surfaces and be sold as a painted end product.
The present invention relates particularly but not exclusively to steel strip that has a metallic coating formed from the above-described molten Al—Zn—Si—Mg alloy composition and is also optionally coated with a paint and thereafter is cold formed (e.g. by roll forming) into an end-use product, such as building products (e.g. profiled wall and roofing sheets).
One corrosion resistant metal coating bath composition that is used widely in Australia and elsewhere for building products, particularly profiled wall and roofing sheets, is a 55% Al—Zn alloy coating composition that also contains Si. The profiled sheets are usually manufactured by cold forming painted, metal alloy coated strip. Typically, the profiled sheets are manufactured by roll-forming the painted strip.
The addition of Mg to this known composition of 55% Al—Zn—Si alloy coating composition has been proposed in the patent literature for a number of years, see for example U.S. Pat. No. 6,635,359 in the name of Nippon Steel Corporation, but Al—Zn—Si—Mg coatings on steel strip are not commercially available in Australia.
It has been established that when Mg is included in a 55% Al—Zn—Si alloy coating composition, Mg brings about certain beneficial effects on product performance, such as improved cut-edge protection.
The applicant has carried out extensive research and development work in relation to Al—Zn—Si—Mg alloy coatings on strip such as steel strip. The present invention is the result of part of this research and development work.
The above discussion is not to be taken as an admission of the common general knowledge in Australia and elsewhere.
The present invention is based on a finding of the applicant during the course of the research and development work that forming an Al—Zn—Si—Mg alloy coating on a steel strip so that there is an intermediate alloy layer having a selected composition and preferably a selected crystal structure between an Al—Zn—Si—Mg alloy coating overlay layer and the steel strip can improve the corrosion performance of the coated strip. The research and development work also found that the selected composition and preferred crystal structure of the intermediate alloy layer that can improve corrosion performance of the coated strip is not an inevitable outcome of the selection of the Al—Zn—Si—Mg alloy composition for use in a hot dip coating bath, and a number of factors such as but not limited to molten Al—Zn—Si—Mg alloy bath composition and hot dip process conditions, typically strip immersion time and coating pot temperature, are relevant factors to forming the intermediate alloy layer having the required composition and the preferred crystal structure.
According to the present invention there is provided a metallic coated steel strip that includes a steel strip and a metallic coating on at least one side of the strip, with the metallic coating including an Al—Zn—Si—Mg overlay alloy layer and an intermediate alloy layer between the steel strip and the overlay alloy layer, and wherein the intermediate alloy layer has a composition of, by weight, 4.0-12.0% Zn, 6.0-17.0% Si, 20.0-40.0% Fe, 0.02-0.50% Mg, and balance Al and unavoidable impurities.
The intermediate alloy layer may be formed as an intermetallic phase of elements in the compositions of the molten Al—Zn—Mg—Si alloy and the steel strip.
Alternatively, the intermediate alloy layer and the Al—Zn—Mg—Si overlay alloy layer may be formed as separate layers.
The intermediate alloy layer may include, by weight, 5.0-10.0% Zn, 7.0-14.0% Si (typically 6.5-14.0% Si), 25.0-37.0% Fe, 0.03-0.25% Mg, balance Al and unavoidable impurities.
The intermediate alloy layer may include, by weight, 6.0-9.0% Zn, 8.0-12.0% Si, 28.0-35.0% Fe, 0.05-0.15% Mg, balance Al and unavoidable impurities.
The intermediate alloy layer may include, by weight, 0.01-0.2% Ca.
The intermediate alloy layer may include, by weight, 0.1-3.0% Cr.
The intermediate alloy layer may include, by weight, 0.1-13.0% Mn.
The intermediate alloy layer may include, by weight, 0.1-2.0% V.
The intermediate alloy layer may have a thickness of 0.1-5.0 μm as measured on a cross-section through the thickness of the coating.
The intermediate alloy layer may have a thickness of 0.3-2.0 μm as measured on a cross-section through the thickness of the coating.
The intermediate alloy layer may have a thickness of 0.5-1.0 μm as measured on a cross-section through the thickness of the coating.
The intermediate alloy layer may include substantially columnar crystals measuring 50-1000 nm in a short diameter as measured on a cross section through the thickness of the coating.
The intermediate alloy layer may include substantially equiaxial crystals measuring 50-4000 nm in a long diameter as measured on a cross section through the thickness of the coating.
The intermediate alloy layer may include a mixture of columnar crystals and equiaxial crystals.
The intermediate alloy layer may include body centred cubic crystals.
The Al, Zn, Si and Fe concentrations of the intermediate alloy layer may satisfy the formula Fe10Al32Si5Zn3.
The Al, Zn, Si and Fe concentrations of the intermediate alloy layer may satisfy the formula Fe10Al34Si4Zn2.
The strip may be a passivated strip, for example using a Cr-containing or Cr-free passivation system.
The strip may include a resin coating on an exposed surface of the Al—Zn—Mg—Si alloy coating.
A molten Al—Zn—Si—Mg alloy for forming the metallic coating may include more than 0.3% by weight Mg.
The molten Al—Zn—Si—Mg alloy for forming the metallic coating may include more than 1.0% by weight Mg.
The molten Al—Zn—Si—Mg alloy for forming the metallic coating may include more than 1.3% by weight Mg.
The molten Al—Zn—Si—Mg alloy for forming the metallic coating may include more than 1.5% by weight Mg.
The molten Al—Zn—Si—Mg alloy for forming the metallic coating may include less than 3% by weight Mg.
The molten Al—Zn—Si—Mg alloy for forming the metallic coating may include less than 2.5% by weight Mg.
The molten Al—Zn—Si—Mg alloy for forming the metallic coating may include more than 1.2% by weight Si.
The molten Al—Zn—Si—Mg alloy for forming the metallic coating may include less than 2.5% by weight Si.
The molten Al—Zn—Si—Mg alloy for forming the metallic coating may include the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
Zn: 30 to 60%Si:0.3 to 3% Mg:0.3 to 10%BalanceAl and unavoidable impurities
In particular, the molten Al—Zn—Si—Mg alloy for forming the metallic coating may include the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
Zn:35 to 50%Si:1.2 to 2.5%Mg1.0 to 3.0%BalanceAl and unavoidable impurities.
The steel may be a low carbon steel.
According to the present invention there is also provided a method of forming a metallic coating on a steel strip to form the above-described metallic coated steel strip, the method including dipping steel strip into a bath of a molten Al—Zn—Si—Mg alloy and forming a metallic coating of the alloy on exposed surfaces of the steel strip, and the method including controlling any one or more of the composition of the molten alloy bath, the temperature of the molten alloy bath, and the immersion time of the steel strip in the molten alloy bath to form the intermediate alloy layer between the steel strip and the Al—Zn—Mg—Si overlay alloy layer.
The molten Al—Zn—Si—Mg alloy may have the composition described above. For example, the molten Al—Zn—Si—Mg alloy may include the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
Zn: 30 to 60%Si:0.3 to 3% Mg:0.3 to 10%BalanceAl and unavoidable impurities.